Apparatus for making a mold for reproducing parts of shoes and the like

ABSTRACT

The apparatus of making a master mold for reproducing parts having raised portions on its face comprising providing a master pattern of the part to be reproduced, displacing the raised portions thereof into a substantially common plane, supporting the master pattern within a mat with the face projecting therefrom by an amount corresponding substantially to the depth of the cavity of the mold to be made, flooding the projecting portion of the master pattern with a curable liquid material having a lower dielectric constant than the thermoplastic of which the part is to be made, curing the liquid material while holding it confined to a predetermined depth above the master pattern and then stripping the cured material from the master pattern, said cured material then comprising the master mold.

atet I191 Prahi, .111. et a Dec. 10, 1974 [75] Inventors: Frederick A.Prahl, Jr., Carlisle;

Robert B. Dunlap, Medway, both of Mass.

[73] Assignee: Compo Industries, Inc., Waltham,

Mass.

[22] Filed: Apr. 4,1973

[21] Appl. No.: 347,850

Related Us; Application Data [62] Division of Ser. No. 173,072, Aug. 19,1971, Pat. No.

3,217,067 11/1965 Tencate 425/175 X 3,422,168 [[1969 Bouser 425/175 X3,581,374 6/1971 Geary 425/175 X 3,635,630 1/1972 Greene 425/1753,773,879 11/1973 Munsil et al 425/175 X Primary Examiner-Robert L.Spicer, Jr. Attorney, Agent, or Firm-G. Eugene Dacey [57] ABSTRACT Theapparatus of making a master mold for reproducing parts having raisedportions on its face comprising providing a master pattern of the partto be reproduced, displacing the raised portions thereof into asubstantially common plane, supporting the master pattern within a mat.with the face projecting therefrom by an amount correspondingsubstantially to the depth of the cavity of the mold to be made,flooding the projecting portion of the master pattern with a curableliquid material having a lower dielectric con-' stant than thethermoplastic of which the part is to be made, curing the liquidmaterial while holding it confined to a predetermined depth above themaster pattern and then stripping the cured material from the masterpattern, said cured material then comprising the master mold.

4 Claims, 21 Drawing Figures PATENTEUBEEIOIQH 3,853,445

SHEET 10F 4 FIGI FIGZ l////////////////////LI PATEHTEB BECI 01974 SHEEI2 [IF 4 MASTER MOLDING DIE FIG? APPARATUS FOR MAKING A MOLD FORREPRODUCING PARTS OF SHOES AND THE LIKE This is a division ofapplication Ser. No. 173,072, now US. Pat. No. 3,751,540, filed byFrederick A. Prahl and Robert B. Dunlap on Aug. 19, 1971.

BACKGROUND OF THE INVENTION Making molds from a master pattern for usein reproducing an exact imitation of the master pattern on sheetmaterial, for example, thermoplastic sheet material is not new andgenerally comprises carefully preparing a master pattern, applying aliquid material to the master pattern, allowing it to set and thenstripping it from the master pattern. Silicone rubber is the preferredmaterial because it takes the configuration of the master pattern inintimate detail and when cured may be easily stripped from the masterpattern without need for parting compounds. In making such molds thecavity must correspond substantially in depth to the thickness of thesheet material which is to be molded otherwise the molding pressure willnot force the surface of the sheet thermoplastic into intimateengagement with the bottom of the mold cavity so that only an imperfectreproduction will be produced. This makes it necessary when attemptingto reproduce surfaces which have dif ferent elevations and levels byreason of overlapping parts, seams, stitching, cut-outs and the like tocarefully and painstakingly prepare the-master pattern by splittingheavy leathers and skiving overlapping parts to try to obtain asubstantially uniform overall thickness corresponding substantially tothe thickness of the thermoplastic material to be molded from which tomake the mold. This requires skill and consumes an. abnormal amount ofshoe making time since a relatively large number of molds are made forthe foregoing purposes, each being prepared, of course, according to thestyle of shoe upper to be made. The purpose of this invention is toeliminate the foregoing by an improved technique which enables makingthe moldfrom the master pattern to a uniform depth correspondingsubstantially to that of the thermoplastic sheet which is to be moldedindependently of the thickness of the material of the master patternand/or differences in thickness thereof by reason of overlays, seams,stitching, perforations and the like and which provides for a simple,quick and relatively inexpensive way of making molds which will producean exact imitation of the configuration of the master mold.

SUMMARY As herein illustrated, the apparatus comprises displacing theelevated portions at the face of a master pattern which is to bereproduced into a substantially common plane, supporting the masterpattern within a mat with the face projecting above the surface thereofa predetermined amount corresponding substantially to the depth of themold cavity to be formed, flooding the portion of the master patternprojecting from the mat with a curable liquid material having a lowerdielectric constant than the thermoplastic of which the part is to bereproduced to a predetermined depth which is greater than the thicknessof the projecting portion of the master pattern, causing the liquidmaterial to cure and then stripping the cured material from the masterpattern and surrounding mat.

According to the preferred procedure, the displacement of the raisedportions of the master pattern is effected by mounting the masterpattern face down on a slightly resilient layer of sheet materialsupported on a rigid platen, placing a mask about the master pattern,bringing a sheet of flexible material into engagement with the back sideof the master pattern, applying a vacuum through holes in the platen tothe face of the master pattern to displace the raised portions on theface rearwardly into the sheet of flexible material to a depth such thatthe portion of the master pattern projecting from the face of theflexible sheet corresponds substantially to the thickness of the maskand, while holding the master pattern embedded within the sheet offlexible material, applying a layer of quick-setting material to theback of the flexible sheet, for example, plaster of Paris and allowingit to solidify to form a rigid backing. The mask is now removed and therigidly backed master pattern is supported with the master patternfacing upwardly whereupon the surface of the master pattern and thesurrounding backing are flooded to a uniformly controlled depth with aroom temperature curing silicone rubber. After the silicone rubber hascured it is stripped from the surface of the master pattern andconstitutes the master molding die.

Alternatively, the displacement of the raised portions of the masterpattern is effected by mounting the master pattern face down on aslightly resilient layer of sheet material supported on a rigid platen,placing a mask about the master pattern, bringing a softened sheet ofplastic material into engagement with the back side of the masterpattern and applying a vacuum through the rigid platen to the face todisplace the raised portions on the face rearwardly into the plasticsheet and to embed the master pattern in the plastic sheet to a depthsuch that the portion projecting from the face of the plastic sheetcorresponds substantially to the thickness of the mask. Having effectedthis displacement the platic sheet is restored to its rigid condition.The rigid sheet of plastic with the master pattern embedded therein isnow placed with the master pattern facing upwardly, is shimmed from theunderside, if necessary, to produce a substantially perfectly levelsurface and then flooded to a uniformly controlled depth with aroom-temperature curing silicone rubber. Following curing the siliconeis stripped from the master pattern and constitutes the formed mold.

According to still another procedure, the master pattern which is to bereproduced is placed with its rear side in engagement with a sheet ofwax or similar impressionable material of larger area whereupon themaster pattern is pressed into the wax sheet in such fashion as to forcethe raised portions of its surface into a substantially common plane andto embed the master pattern in the wax sheet so that the surrounding waxsheet becomes a supporting mat for the master pattern. The masterpattern may be pressed into the surface of the wax so that only the faceprojects therefrom by the amount corresponding to the depth of thecavity of the mold to be made or a mask of that thickness may be placedon the wax sheet above the master pattern. Usually an adhesive isemployed between the master pat tern and the wax to effect adhesion. Thecomposite structure comprising the master pattern with the wax backingis then supported with the master pattern facing upwardly whereupon thesurface of the master pat- The invention will now be described ingreater detail with reference to the accompanying drawings wherein:

FIG. 1 is a fragmentary section of the master pattern of the shoe partwhich is to be reproduced, which has raised portions on its surface byreason of overlapping parts;

FIG. 2 shows the relation of the surfaces at the face of the masterpattern achieved by flattening;

FIG. 3 diagrammatically illustrates the preferred pro-' cedure forflattening the master pattern and embedoing it in a supporting mat,prior to such flattening;

FIG.'4 shows the flattening of the master pattern preparatory toapplication of a rigid, supporting backing thereto of a quick-settingmaterial which will form a rigid structure, for example, plastic ofParis;

FIG. 5 shows the master pattern embedded in the rigid, plaster of Parisbacking with the mask removed and with its face projecting therefrom anamount corresponding substantially to the depth of the mold to be made;

FIG. 6 shows the master pattern in the plaster of Paris backing with themaster pattern facing upwardly therefrom and flooded withroom-temperature, curable, silicone rubber;

FIG. 7 shows the completed master mold stripped from the master pattern;

FIG. 8 diagrammatically illustrates an alternative procedure forflattening the master pattern and embedding it in a rigid supportingbacking comprised of a sheet of a normally rigid plastic, prior .to suchflatten- FIG. 9 shows the flattened master pattern in engagement withthe softened plastic backing sheet;

FIG. 10 shows the master pattern embedded in the plastic backing sheetafter hardening with the mask removed and with its face projectingtherefrom an amount corresponding substantially to the depth of the moldto be made;

FIG. 11 shows the supporting plastic backing sheet supported with themaster pattern facing upwardly with its face flooded withroom-temperature silicone rubber;

FIG. 12 shows a fragmentary section of the shoe upper or other articlewith a raised portion comprising, for example, an overlay; v

FIG. 13 shows the desired displacement of the overlay into the face toprovide for a substantially flat surface;

FIG. 14 shows a section of a shoe upper or other article provided withseams; I

FIG. 15 shows the displacement of the seams into the surface to providea substantially flat surface;

FIG. 16 is an elevation of an apparatus for embed ding the masterpattern in a mat according to either of the procedures illustrated inFIGS. 3 to 5 or FIGS. 8 to FIG. 17 is an elevation, partly in section,of an apparatus for supporting the plastic backing with the masterpattern in an upwardly facing position specifically illustrating theleveling process; 5 FIG. 18 shows the flooding of the surface of themaster pattern with silicone rubber; and

FIGS. 19, 20 and 21 illustrate alternative methods by means of which theraised portions are manually displaced to form supporting structures formaster patterns'from which master molds may be made.

According to the preferred method of making, the mold a master patternof the part to be reproduced, such as shown at 10 (FIG. 1) comprising acomponent 12 and overlapping components 14-14 is placed face down (FIG.4) upon a sheet of slightly resilient material 16 resting on a rigidmetal plate 18 containing perforations 20, the latter in turn resting ona supporting platen 54. A mask 22 of greater area than the masterpattern and containing an opening 24 corresponding in placed over themaster pattern so that it rests on the sheet of resilient material 16.The assembly comprising the resilient material 16, the metal plate 18,the master pattern 10 and the mask 22 is then brought into juxtapositionwith a thin flexible sheet of impervious'material 26 of greater areathan the mask and a vacuum is applied to the under-side of the metalplate 18. As a result of the application of the vacuum to the face ofthe master pattern the component portion 12 on the face is displaceddownwardly so that its surface lies substantially in the same plane asthe surfaces of the portions 1414, as shown in FIGS. 2 and 4, therearwardly displaced portions being taken up in the impervious sheet.The vacuum draws the impervious sheet about the back side of the masterpattern against the mask, and about the edges of the resilient material16 and plate 18 into engagement with the supporting platen 54 embeddingthe master pattern in the sheet and forming a-surrounding wall 27 andflange 29 around the master back 'side of the master pattern and theimpervious sheet it is desirable to apply an adhesive to the back sideof the master pattern.

7 While the master pattern is held embedded in the impervious sheetasshown in FIG. 4, a relatively .thick layer of quick-setting materialwhich when hardened will form a rigid structure such as plaster of Paris38 is applied to the rear side of the impervious sheet and allowed toharden. If desired, an adhesive maybe applied to the rear surface of theimpervious sheet prior to the application of the plaster of Paris. Theimpervious sheet employed may be comprised of a relatively thinopen-mesh woven fabric, coated on one side, such as oil cloth or itsequivalent. After the plasterof Paris is hardened the vacuum isreleased, the platen lowered and the rigid plaster of Paris backed sheetwith the master pattern embedded in it unclamped and removed. The maskis now removed leaving the master pattern projecting from the surface ofthe surrounding plaster of Paris backed sheet.

. According to the method herein illustrated, the metal plate 18 and theresilient sheet 16 have a combined thickness corresponding to theultimate thickness desired at the bottom of the mold to be formed sothat when the impervious sheet material is drawn about the edges of themaster pattern, the mask, the resilient configuration to the peripheraledge of the part is pattern. In'order to ensure a goodbond between thesheet and the plate and backed by the plaster of Paris, the resultingmat has a surrounding wall 27 terminating in a flange 29 providing acavity surrounding the master pattern, the depth of which from the faceof the master pattern to the flange corresponds to the aforesaid desiredthickness for the mold bottom. By thus predetermining and controllingthe depth of the cavity relative to the face of the mat a uniformthickness may be assured for the bottom of the mold and hence uniformdielectric charcteristics throughout the bottom of the mold over theentire area of the mold surface.

The plaster of Paris backing, which forms a supporting mat for themaster pattern, is now employed to support the master pattern facingupwardly as shown in FIG. 6, whereupon a curable liquid material 32having a lower dielectric constant than the thermoplastic of which thepart is to be reproduced is poured into the cavity to completely fillit, covering the upper surface of the master pattern to a depthcorresponding to the depth of the cavity whereupon a plate 34 containingholes 36 is placed on the flange 29 of the upper edge of the wall 27 toconfine the liquid material during curing. In accordance with customarypractice an excess of liquid 32 is poured into the cavity so that whenthe plate 34 is placed against the flange to cover the cavity enoughpressure is applied to level the material within the cavity and to causesuch excess as there is to rise in the holes within the cover plate 34.This ensures uniform depth and hence the uniform thickness desirable.After curing any of the excess cured material which extrudes through theriser holes is cut off, the plate removed and any nubs on the surface ofthe mat carefully removed. The resulting cured structure comprises themaster mold as shown in FIG. 7.

Alternatively, the method comprises, as shown in FIGS. 8, 9 and 10,placing the master pattern 10 face down on a sheet of resilient material16 resting on a rigid metal plate 18 containing perforations 20, placinga mask 22 of greater area than the master pattern and containing anopening 24 corresponding in configuration to the peripheral edge of themaster pattern on the sheet of resilient material 16- and then raisingthe assembly into juxtaposition with a softened sheet of plastic 26a ofgreater area than the mask and applying a vacuum to the underside of themetal plate through the openings 20. The plastic sheet is softened by aheating grid 46. Application of the vacuum to the face of the masterpattern displaces the raised portions rearwardly so that the surfacelies substantially in the same plane, the rearwardly displaced portionsbeing taken up in the plastic sheet. The vacuum embeds the back side ofthe master pattern and draws the sheet about the edges of the masterpattern, mask, resilient sheet and plate into engagement with thesupporting platen forming, as related above, a surrounding wall 27 andflange 29 defining an open-top cavity around the master pattern.

Following application of the vacuum to draw the softened sheet plasticabout the master pattern and while the vacuum is held and upon coolingthe softened plastic sheet is restored to its rigid condition whereuponthe mask is removed, leaving the master pattern embedded in the rigidsheet surrounded by the extending portion of the sheet which comprises asupporting mat, as shown in FIG. 10.,

In order to ensure good bonding between the back.

side of the master pattern and the plastic sheet, itis de sirable toapply an adhesive to the back side of the master pattern which iscompatible with the plastic sheet.

Instead of applying a vacuum to the face of the master pattern, airpressure may be applied to the back of the mat to efiect flattening.

The mat is now employed to support the master pattern 10 facingupwardly, as shown in FIG. 11, on a support 40 whereupon a curable,liquid material 30 having a lower dielectric constant than thethermoplastic of which the part is to be reproduced is poured into thecavity to completely fill the cavity and cover the upper surface of themaster pattern 10 to a depth corresponding to the depth of the cavitywhereupon a flat plate 32 containing holes 34 is placed on the flange 29surrounding the wall 27 to constrain the liquid material while curing.When the liquid material is cured the plate 32 is removed, the curedmaterial is stripped from the surface of the master pattern and thesurrounding mat and comprises, as shown in FIG. 7, a completed mastermold. The holes in the plate provide risers through which the excessliquid material may expand during curing and the nubs formed in theserisers are removed to provide a perfectly flat back surface.

In the manufacture of the mold for reproducing parts of sheetthermoplatic, it is desirable that the mold cavity be of somewhat lesserdepth than the thickness of the material being molded. Accordingly, themask 22 is comprised of a material which is somewhat less than thethickness of the part to be molded so that when the master patternbecomes embedded in the backing sheet with the plaster of Paris, theplaster not only embeds the rearwardly displaced portions but also theportions of the peripheral edge projecting rearwardly from the mask,leaving a portion projecting from the face of the mat which correspondsto the thickness of the mask which, as related above, is comprised of amaterial, the thickness of which is slightly less than the thickness ofthe thermoplastic sheet material of which the part is to be molded sothat the mold cavity will have a depth less than the thickness of thepart to bemolded.

FIGS. 12, 13, 14 and 15 show examples of parts having raised areas whichmay be treated in the foregoing fashion; FIG. 12, for example,comprising an overlay and FIG. 14 seams. Other surface projections maytake the form of stitches, appliques, perforations, bindings, eyeletstays, grain and the like.

The molding die made according to either procedure may be used forreproducing parts from thermoplastic sheet materials, but preferably isused as a master mold from which a production mold is made comprised ofa material which has a greater resistance to wear than the siliconerubber of which the master mold is comprised.

- To this end, a negative of the master mold is made and from thisnegative a production mold or number of production molds are made.

As related above, silicone rubber is the most desirable material formaking the master mold, it is to be understood, however, that othermaterials may be employed, the criterion being that the material musthave a lower dielectric constant than the sheet thermoplastic of whichthe part is to be reproduced. Materials which may be employed in lieu ofsilicone rubber are polyurethane and polysulfide rubber. The chiefadvantages of silicone rubber in carrying out the process hereindescribed is that it will not stick to anything but itself and hencethere is no problem with sticking of the silicone rubber to the matand/or to the master pattern. Other advantages provided by siliconerubber are good detail, room-temperature curing and the desireddielectric characteristics.

An apparatus for carrying out the procedures related above wherein themaster pattern is embedded in a plaster of Paris backing mat or in aplastic backing mat is shown in FIG. 16 comprising a supporting frame 41mounting a clamping ring 43 between which the fabric sheet or theplastic sheet is adapted to be clamped and supported across a topopening 42. When the backing mat is to be comprised of plastic, aheating grid 46 is supported by means of a bracket 44 above the clampingring so that it may be swung from an out of the way position to aposition above the clamping ring and comprises radiant heating elements48 which when energized will by radiation soften the plastic sheet.Within the supporting frame there is mounted a fluid cylinder 50provided with a ram 52 at the upper end of which there is a platen 54containng a vacuum chamber 56,

the upper side of which contains transverse longitudinal channels 58connected by an opening 60 to the interior of the chamber. A flexibleconductor 62 is connected to the lower side of the chamber to applyvacuum to the interior.

With this equipment the initial step comprises placing the flat metalplate 18 on the surface of the platen so that the holes therein are incommunication with one or more of the channels 58. The sheet 16 ofresilient material is now placed facedown on the metal plate and thiscomprises preferably a sheet of split chrome leather, although otherkinds of sheet material may be employed, for example, felt or rubber.The master patbe made and this is slightly less than the thickness ofthe thermoplastic material which is to be molded. The mask 22 may becomprised of metal or of a plastic material which does not becomeplastic at the temperature employed in softening the plastic sheet whenthe latter is used to form the backing mat.

When making the mat, if the mat is comprised of plaster of Paris, theplaten 54 is raised to bring the master pattern into engagement with theflexible impervious sheet 26 whereupon a vacuum is applied to the vacuumchamber through the conductor 62. The vacuum draws the flexibleimpervious sheet into intimate contact with the rear side of the masterpattern and about the peripheral edge thereof against the mask and aboutthe edges of the resilient sheet 16 and plate 18 into engagement withthe supporting platen 54 forming a'surrounding wall 27 and flange 29,the wall defining a cavity, the depth of which from the face of themaster pattern corresponding to the thickness of the plate 18 andresilient material 16. At this point and while maintaining the vacuum, aring is placed about the assembly as shown in FIG. 4 whereupon plasterof Paris is poured into the ring over the exposed rear surface of theflexible impervious sheet to form a layer of greater thickness than therearwardly displaced portion of the master pattern and allowed toharden. As previously indicated, an adhesive may be applied to the backof the master pattern and also the back of the flexible impervioussheet. Following hardening of the plaster of Paris, the ring 55 isremoved, the clamping ring 43 is released and the rigid plaster of Parismat with the master pattern embedded therein removed. The plaster ofParis mat may now be employed, as previously described, to make themaster silicone mold of FIG. 7.

When the mat is to be comprised of sheet plastic material the sameprocedure is followed as to assembling the parts on the platen. However,in this instance the grid 46 is swung to a position above the plasticsheet to soften it whereupon the platen is raised to bring the masterpattern into engagement with the softened sheet and the vacuum applied.As soon as the plastic sheet has been conformed to the rear side of themaster pattern and drawn about the edges of the resilient sheet and theplate into engagement with the supporting platen by the vacuum, theheater is swung to an out of the way position and water is sprayed overthe surface of the sheet plastic to cool it and cause it to becomerigid. Other means could be used, for example, a blast of refrigeratedair. When the sheet plastic is completely rigid, the vacuum is released,the platen lowered and the rigid plastic backing sheet with the masterpattern and mask embedded in it unclamped and removed. The mask is nowremoved leaving the master pattern projecting from the surface of thesurrounding plastic backing sheet which now becomes a supporting mat. Aspreviously indicated, a suitable adhesive is applied to the rear side ofthe master pattern before it is brought into engagement with thesoftened plastic sheet. g

It is usually desirable to trim off the portion of the plastic sheetwhich was held between the clamping means 43 after the sheet is removedbecause softening and re-rigidifying produces stresses and strains inthe plastic sheet which tend to cause the mat to become distorted andnot to lie perfectly flat.

Following trimming the supporting mat with the master pattern embeddedin it is supported by the flange 29 across a rigid frame 64, the latterin turn being supported by. a plate 65 as shown in FIG. 17. The plate 65is supported on legs and has threaded through its bottom a plurality ofadjustable shim screws 72. A ring 76 is clamped down on the flange byclamping elements 74 and tomake sure that the surface of the masterpattern is perfectly level, a rigid bar 78 is placed across the ring 76and a dial indicator is mounted thereon for movement across the masterpattern to determine the level. The bar 78 may be moved about bothtransversely and longitudinally of the ring to test the flatness of theentire area. The shin screws 72 are accessible from the lower side ofthe plate and by adjustment, as indicated by the dial reading, enablebringing the entire surface into a substantially common plane.

Having leveled the master pattern, a curable liquid material having alower dielectric constant than the thermoplastic of which the part is tobe made, for example, room-temperature curing silicone rubber, is pouredonto the mat (FIG. 18) to flood the surface of the master pattern andthe surrounding mat to a depth corresponding to the depth of the cavitywithin the surrounding wall 27. The plate 32 is clamped across theflange at the open top of the mat to constrain the silicone rubber whilecuring takes place and preferably contains one or more holes 34 to allowfor escape of excess material during the curing process. As soon as thecuring takes place the clamping ring is removed and any excess materialwhich extrudes through the holes trimmed off, whereupon the curedsilicone rubber is striped from the master pattern. The nubs formedwithin the holes are now trimmed off the back and the resultingstructure comprises the master mold of FIG. 7.

The vacuum process related above makes it possible to reproduce shoeuppers prepared in the shoe factory in conventional fashion without needfor careful selection of stock, splitting, skiving and like operationssince it is independent of the thickness of the material of the masterpattern.

While the foregoing procedures are the most expedient there arealternatives. Thus, for example, as shown in FIG. 19, the master patterna may be placed face side up on a sheet of wax 88, for example,caranauba wax or similar impressionable material and pressed into theface of the wax sheet as shown in FIG. 20, so as to displace the edgesof the part 12 into the sheet and to bring the parts 1414 into the planeof the part 12 between them. A suitable adhesive is used to cause theparts to adhere firmly to the wax, the latter, of course, being ofsufficient rigidity to hold the parts in a substantially flat plane. Theface of the master pattern projects from the wax sheet an amountcorresponding substantially to the depth of the mold to be made and thesurrounding portion of the wax sheet comprises a supporting mat. Thecomposite structure comprising the master pattern and its supporting matis supported on a flat surface within a ring and the mat and surface ofthe master pattern are flooded with a curable liquid material having alower dielectric constant than that of the thermoplastic of which thepart is to be made, preferably room-temperature curing silicone rubber.While the silicone rubber is curing it is confined within the ring by arigid plate to provide for a uniform predetermined depth. When thesiliconerubber is cured it is stripped from the master pattern and themat and comprises the master mold.

Another alternative is to employ, instead of a single layer of wax 88, anumber of strips of wax 90 and to build up the back of the masterpattern as shown, for example, in FIG. 21, by filling the recesses 92,94 and 96 at the back side with such strips to form a substantially flatback and then supporting the composite structure within a ring to makethe mold in the same fashion described with reference to FIGS. 19 and20.

produce a mold with a cavity of a predetermined depth independently ofthe thickness or differences in thickness of the master pattern and toproduce a mold, the bottom of which beneath the mold surface isuniformly thickened so as to afford uniform dielectric properties duringmolding. It is also to be noted that by providing all molds with thesame bottom thickness the need for adjusting the high frequencygenerator for each molding operation is eliminated.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents falling within the scope of the appended claims.

We claim:

1. An intermediate mold for molding making a portion of a master moldwhich, in turn, is used to reproduce from a master pattern a partcomprised of thermoplastic material simulating the master pattern,comprising a flexible sheet which is of impervious material and set in alayer of quick-setting material which forms a rigid backing for theflexible sheet, said impervious material and quick-setting materialbeing united to provide a rigid mat into which also is embeded themaster pattern to a depth such that the portion of the master patternprojecting from the mat corresponds substantially to the thickness ofthe thermoplastic sheet material to be molded and a surrounding walldefining a cavity corresponding in depth from the surface of the masterpattern to the thickness of the bottom of the mold to be made therefrom.

terial is a sheet of wax.

1. An intermediate mold for molding making a portion of a master moldwhich, in turn, is used to reproduce from a master pattern a partcomprised of thermoplastic material simulating the master pattern,comprising a flexible sheet which is of impervious material and set in alayer of quick-setting material which forms a rigid backing for theflexible sheet, said impervious material and quick-setting materialbeing united to provide a rigid mat into which also is embeded themaster pattern to a depth such that the portion of the master patternprojecting from the mat corresponds substantially to the thickness ofthe thermoplastic sheet material to be molded and a surrounding walldefining a cavity corresponding in depth from the surface of the masterpattern to the thickness of the bottom of the mold to be made therefrom.2. A mold according to claim 1, wherein the rigid material is plaster ofParis.
 3. A mold according to claim 1, wherein the rigid material is asheet of normally rigid plastic.
 4. A mold according to claim 1, whereinthe rigid material is a sheet of wax.